Alkylglycosides, which are sugar-derived surfactants, are surfactants less irritant to skin.
One of the conventional processes for producing alkylglycoside is a method wherein sugar and higher alcohol having not less than 8 carbon atoms are directly reacted in the presence of an acid catalyst (hereinafter referred to as "direct method"), and many inventions have been made according to this method. In such direct method, coexistence of sugar and water with an acid catalyst results in a reduced yield and deterioration in color due to by-products such as higher condensate of sugars (hereinafter referred to as "polysaccharides"), which is undesirable. Accordingly, anhydrous sugars free from water such as crystallization water have been generally used as sugar sources.
However, solid sugars containing crystallization water or solid sugars having water absorbed onto their surface (hereinafter referred to as "hydrated solid sugars") are cheaper and economically more advantageous than anhydrous sugars. However, when hydrated solid sugars are used, dehydration should be previously conducted before addition of an acid catalyst because coexistence of water is disadvantageous as mentioned above. As literature describing such dehydration of hydrated solid sugar, for example, U.S. Pat. No. 4,950,743 discloses a method wherein higher alcohol is mixed with hydrated solid sugar, which is kept at high temperature and at low pressure while dehydration is carried out before addition of a catalyst until water content of the mixture falls in the range from 0.1 to 0.25% by weight. WO 90/06933 relates to a process for producing an glycoside with excessive sugars, which discloses that when hydrated solid sugar is used, such sugar is heated at elevated temperature under reduced pressure and dehydrated until water content of the mixture becomes lower than about 0.1% by weight. In addition, WO 93/24504 discloses a method wherein a part of alcohol is previously mixed with hydrated solid sugar, and the mixture is introduced to the rest of alcohol, which had been heated, at a controlled flow rate, then heated at the temperature, at which the sugar becomes syrup, or lower, to dehydrate the mixture until the water content of the mixture becomes 0.5% by weight, after which an acid catalyst is introduced.